A split-flow mode modified-Claus plant currently in operation is capable of processing a twofold range of acid gas flows. In light of a predicted decline in the inlet flow, a turndown in excess of the existing limit is desired. The process changes to the Claus plant which will decrease the minimum processing capacity are described. Results are presented which indicate that these changes will derive a more efficient process in regard to heat conservation and sulphur recovery. Solutions to difficulties which may arise in other Claus-type sulphur recovery units of the split-flow kind, when processing acid gas at flows much reduced from design, are also discussed.


Inherent to the processing of a nonrenewable resource is the inevitable requirement of handling flows much reduced from design. Under low flow conditions, the situation will ultimately occur where the minimum processing limit of a facility is met (this limit below which efficient operation of a process can no longer be maintained is often expressed in terms of the turndown ratio – defined as the ratio of maximum to minimum flow l). If the process is operating at this minimum limit, steps should be taken to restore efficient operation or else production from the unit may have to be curtailed.

The work reported in this paper will outline the procedure followed to identify the minimum processing capacity of an existing split-flow mode sulphur plant, detail the modifications required to reduce this minimum flow limit, and present solutions to additional problems which may occur when other modified Claus plants of the split-flow type are processing flows much reduced from design.


The existing split-flow type Claus plant is a two converter process with two sulphur condensers, one hot gas bypass reheat and one gas to gas heat exchange reheat (Figure 1). A "lean " H2S – bearing stream (the mole percent H2S in the acid gas for a split-flow process 2 generally ranges between 20 and 50) flows out of a liquid knockout drum and, prior to entering the burner of the reaction furnace, a portion of this stream is "drawn off" and bypassed. The stream flowing to the furnace is subsequently burned in the presence of air in order to oxidize a portion of the H2S to SO2 (Figure 2): Chemical equation (Available in full paper)

The exothermic nature3 of Reaction 1 yields a temperature of the combustion products out of the varies significantly depending upon the quantity of inlet gas which is bypassed2. The correct amount of bypass depends primarily on the experience gained in the operation of each individual plant. In relation to the unit under study, the amount of acid gas which is considered optimum to bypass is about 40% of the inlet stream. Computer simulation of the process indicates that with bypass amounts approaching the theoretical maximum of 66%, significant quantities of NO begin to form due to higher flame temperatures being sustained.

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